Han, H & Dusseault, MB 2008, 'Formation and Casing Shear During Injection and Production Activities', in Y Potvin, J Carter, A Dyskin & R Jeffrey (eds), SHIRMS 2008: Proceedings of the First Southern Hemisphere International Rock Mechanics Symposium, Australian Centre for Geomechanics, Perth, pp. 261-274, https://doi.org/10.36487/ACG_repo/808_43 (https://papers.acg.uwa.edu.au/p/808_43_Dusseault/) Abstract: We present an analysis of casing shear in China’s largest oil field, Daqing oilfield. Interface slip is the result of pressure differences between different areas, combined with mechanical and transport property differences in the various lithological units. These differences lead to differential strains that in turn cause high shear stress concentrations on lithological boundaries. If pressure differences and mechanical property differences are large enough, an issue of scale and nature of the heterogeneity, formation instability in the form of large-area slip can take place. In addition, there is another source of formation instability in some circumstances. Water-sensitive shales often form the cap rocks to reservoir zones in Daqing, and as water under high pressure is displaced into the formation, a degradation of shale properties takes place, resulting in a weakening of the shale-sand interface. Finally, during high-pressure injection processes, there is a loss of shear strength associated with increased injection pressure, which reduces the frictional strength component. Strong evidence exists that all of these processes took place simultaneously in a region of Daqing oilfield that evidenced formation instability and casing shear. Furthermore, the method of analysis outlined here shows that these processes can be quantified reasonably well, even under the conditions of heterogeneity and massive uncertainty associated with deep processes in oilfield development.